Binding characteristics and functional antagonism exerted by two structurally related
tachykinin NK1 receptor antagonists,
MEN 11467 ((1R,2S)-2N[1(H)indol-3-yl-carbonyl]-1-N-{Nalpha(p-tolylacetyl+ ++)-Nalpha(methyl)-D-3-(2-naphthyl)alanyl}diaminocyclohexane) and
FK888 (N2-[(4R)-4-hydroxy-1-(1-methyl-1H-indol-3-yl)carbonyl-L-prolyl]-N-methy l-N-phenylmethyl-L-3-(2-naphthyl)alaninamide), were investigated in the human
astrocytoma cell line U373 MG. In radioligand binding studies, conducted with [3H]
substance P and intact cells at 37 degrees C,
MEN 11467 bound to
tachykinin NK1 receptors in an irreversible manner with a Ki value of 1.2+/-0.5 nM while
FK888 bound in competitive manner with a Ki value of 4.6+/-2.2 nM. Receptor blockade by both antagonists resulted in a powerful and complete inhibition of functional responses induced by
substance P, such as accumulation of the second messenger
inositol monophosphate or
interleukin-6 release. However,
MEN 11467 showed a greater potency for blocking functional responses than
FK888 despite their similar affinity for human
tachykinin NK1 receptors. Moreover,
MEN 11467 was more potent in inhibiting late rather than early phases of
substance P-induced
inositol monophosphate accumulation and its antagonism was enhanced by
drug preincubation and barely affected by removal of unbound
drug from the external medium, suggesting that
MEN 11467 bound in a tight manner to the receptor. Such behaviour was not observed with the competitive and rapidly reversible antagonist
FK888. These data indicate that the small differences in the chemical structure of
MEN 11467 and
FK888 determine the different binding characteristics at the
tachykinin NK1 receptor and which are responsible for the greater potency of
MEN 11467 for blocking functional responses. The Ki value obtained in binding studies may be inadequate to reveal the real potency of
tachykinin NK1 receptor antagonists.